The present invention relates to a spool for shipping filaments of material having a low thermal coefficient of expansion (TCE), such as thin glass optical waveguide filaments.
The characteristics of a class of optical waveguide filaments and methods of making the same are disclosed in the publication "Doped-Deposited-Silica Fibers for Communications" by R. D. Maurer, Proc. IEEE, Vol. 123, No. 6, June, 1976, pp. 581-585. The tensile strength of such filaments is generally greater than 25,000 psi. However, since the diameters thereof are generally in the range of between 110-140 .mu.m, they cannot be subjected to undue tensile force during handling and shipping.
The temperature variations to which such filaments have been subjected during shipping have caused filament breakage or entanglement as the encountered temperatures were higher or lower, respectively, than those encountered during the filament winding process. Since commonly employed high silica-content filaments have expansion coefficients in the range of 7-15.times.10.sup.-7 /.degree.C., both of these modes of failure were due to the wide variance in thermal expansion rates between the filament and the spool, which was commonly made of polystyrene. This variance caused excessive stress to be transmitted to the filaments at high temperatures and complete loss of winding tension at low temperatures.
U.S. Pat. Application Ser. No. 903,001 entitled "Spool for Filament Winder" filed May 4, 1978 (R.T. Bonzo) teaches a filament winding spool comprising an expanded polystyrene barrel surrounded by a layer of foam rubber which is covered by a thin plastic layer. This spool has been satisfactorily employed in the manufacture of glass optical waveguide filament since temperature is controlled during the manufacturing operation. The foam rubber layer was employed to cushion the filament against roughness of the polystyrene barrel. Since the TCE of polystyrene is about 700.times.10.sup.-7 /.degree.C., this winding spool cannot be used to ship such glass filament under conditions where temperature extremes are encountered.
It was initially thought that the aforementioned disadvantages could be remedied by forming the shipping spool from a material having a thermal coefficient of expansion approaching that of the filament material. If the filament were to be directly wound on the spool barrel, the maximum permissible TCE of the barrel material would be 30.times.10.sup.-7 /.degree.C. Also, the surface of such a barrel should be free from roughness which could introduce microbends into the filament. No satisfactory material could be found.